Detectors are used, for example, in computer tomographs, angiography machines or radiography equipment to convert X-ray radiation into electrical signals which act as the basis for calculating two- or three-dimensional slice images of a patient to be examined. Scintillators are often used for detecting X-ray or gamma radiation. Scintillators are used, in particular, in medical X-ray imaging in an energy range up to 1 MeV.
What are referred to as indirectly converting detectors, what are known as scintillator detectors, are conventionally used, in which the X-ray or gamma rays are converted into electrical signals in two stages. In a first stage the X-ray or gamma quanta are absorbed in a scintillator element and converted into optically visible light. This effect is called luminescence. The light excited by luminescence is then converted in a second stage by a first photodiode optically coupled to the scintillator element into an electrical signal, read out by an electronic evaluation or readout device and then forwarded to an arithmetic unit.
In many applications, for example in computerized tomography or angiography, very high photon fluxes are used to achieve very fast imaging, for example of moving organs. A change in the response function of the detector can occur in the case of high photon fluxes. This change is often called drift. Passivation of light centers occurs as a result of irradiation with X-ray or gamma radiation, wherein the light centers are put into electronic states in which they can no longer contribute to luminescence. The electronic states can dissipate again with time. The electronic states can dissipate with different time constants which can be in the region of a few seconds to several days. Temporarily there is a changed response function which brings about artifacts and/or inaccurate quantitative scans in the imaging. Changes in the response function of the detector can therefore occur within a scan as well as after a large number of scans.
The state of the scintillator element can also have an effect on the transmission properties with some scintillator materials, for example Gd2O2S (GOS). In particular, the transmission of a specific wavelength can be changed under the influence of X-ray radiation. Clouding of the scintillator material can occur, and this can lead to reduced transmission. With other scintillator materials, for example CsI, the transmission can be increased under the influence of X-ray radiation.
Previously calibrations have been performed without object or with a known object suitable for calibration purposes between radiation source and detector in order to ascertain changes in the response function. In this case the radiation source is operated with known parameters and the detector response measured. The changes in the response function of the detector can be used for correcting scan values or for correction during image reconstruction. The use of X-ray or gamma radiation is required for this method. Calibration cannot take place during patient treatment therefore, for example between scans, during a change of patient or immediately before or after a scan. This method of calibration is therefore typically carried out daily. Despite these calibrations artifacts can occur in the imaging and inaccurate quantitative analyses can occur since changes in the response function of the detector with time constants of less than one day cannot be taken into account.